The invention is broadly applicable to joining glass, metal and ceramic components. However, it is particularly applicable to producing envelopes for cathode ray tubes, and the description is so directed.
It is customary in producing cathode ray tube envelopes to press funnel and faceplate components separately. These components are then joined with a fusion seal employing a mid-temperature sealing glass frit.
Lead-zinc-borate sealing glasses, both crystallizing and non-crystallizing, have been used commercially for this purpose over a long period of time. These glasses have proven very successful for the purpose. However, there has been a continuing search for a sealing material that would retain all of the attributes of the lead glasses, but would further improve on some of their features.
A driving force in this search has been a desire for a glass having an even lower sealing temperature than the lead-zinc-borate type glass. Such a lower sealing temperature would be more compatible with thermally sensitive components and coatings present in electronic products such as cathode ray tubes. More recently, the search has been accelerated by the desire to eliminate lead for health and safety reasons.
The materials search led to development of tin-zinc-phosphate glasses as described in U.S. Pat. Nos. 5,246,890 (Aitken et al.) and 5,281,560 (Francis et al.). The glasses described in these patents are lead-free, and provide somewhat lower sealing temperatures in the range of 400.degree.-450.degree. C.
The Aitken et al. glasses are of particular interest for use in producing seals in cathode ray tube envelopes because of their relatively low tin oxide contents. These glasses are lead-free and have compositions containing 25-50 mole % P.sub.2 O.sub.5 and SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range of 1:1 to 5:1. The glass compositions may further contain up to 20 mole % of modifying oxides including up to 5 mole % SiO.sub.2, up to 20 mole % B.sub.2 O.sub.3, up to 5 mole % alkaline earth metal oxide, and up to 5 mole % Al.sub.2 O.sub.3. They may also contain one or more crystallization promoters selected from 1 to 5 mole % zircon and/or zirconia and 1-15 mole % R.sub.2 O. Additionally, the composition may include a seal adherence promoter selected from up to 5 mole % WO.sub.3, up to 5 mole % MoO.sub.3, up to 0.10 mole % Ag metal and mixtures.
In producing a sealing material, mill additions to the sealing glass may be made in amounts up to about 30% by weight with no more than 15% being preferred. These additions are made to provide a sealing material having a lower effective coefficient of thermal expansion (CTE). The mill additions include metal pyrophosphate crystalline materials, cordierite, solid solutions of beta-spodumene or beta-eucryptite, silica and quartz glasses and Invar.
The manufacturing process for cathode ray tubes imposes severe restraints on a flit intended for use in sealing envelope components. One such restraint arises from the need to conduct the sealing operation at temperatures below 450.degree. C. Higher temperatures would exceed the strain point of the funnel glass. This requirement, in turn, necessitates that the viscosity of a sealing frit must be in the range of 10.sup.2 -10.sup.3 MP.multidot.as (10.sup.3 -10.sup.4 poises) in the temperature range of 440.degree.-450.degree. C. Otherwise, the frit will have insufficient flow to form a seal with a strong hermetic bond.
Following the sealing operation, the panel-funnel assembly is reheated under vacuum to a temperature in the range of 300.degree.-400.degree. C. in an exhaust bake-out process. This bake-out removes volatile constituents of the electronic system. It establishes the needed vacuum level in the tube to assure long tube life. The frit requirement for this second step in the process is essentially the opposite of that needed for successful sealing. To survive the exhaust bake-out, the frit must be rigid at exhaust temperatures. This requires a minimum viscosity of 10.sup.8 MP.multidot.as (10.sup.9 poises) to avoid movement in the seal and resulting breakage or loss of vacuum.
These dual viscosity/temperature requirements are met currently by employing high lead frits in the PbO--ZnO--B.sub.2 O.sub.3 system that form a crystallized seal. These lead frits are initially vitreous, but have a small amount of zircon or alumina added as a mill addition to induce crystallization. The frits exhibit excellent flow during the initial portion of the hold at the 440.degree.-450.degree. sealing temperature. Near the end of this hold period, they undergo rapid crystallization to a degree greater than 95%. This forms a strong, rigid seal which remains rigid during the exhaust bake-out process.
Frits in the SnO--ZnO--P.sub.2 O.sub.5 ternary system exhibit good flow properties at temperatures as low as 360.degree. C. They also have expansion coefficients close to the 95-100.times.10.sup.-7 /.degree.C. range characteristic of current panel and funnel glasses. However, the glass frits are relatively resistant to crystallization. While it is possible to crystallize them with a combination of additives, the extent of crystallization is relatively low. Consequently, the crystallized material behaves essentially as a vitreous frit. As a result, these frits form good seals, but have not been successful in surviving the exhaust process.
Currently, there are two schools of thought relative to the bakeout process. Traditionally, bakeout temperatures close to 400.degree. C. have been required. However, presently it is thought that bakeout temperatures below 350.degree. C. may be satisfactory. The present invention is predicated on adoption of the lower temperature bakeout.
My related application, Ser. No. 08/221,400, addresses the problem with a sealing material consisting essentially of a 60-90% SnO--ZnO--P.sub.2 O.sub.5 glass frit and 10-40% of a mill addition including 10-30% alumina and 0-30% zircon. The mill addition causes the sealing material to undergo a substantial change after formation of a seal. Specifically, the seal does not undergo a decrease in viscosity when the seal is reheated. As a result, a seal exhibits non-viscoelastic behavior and the viscosity remains relatively constant up to a bakeout temperature of 380.degree.-400.degree. C. so that the seal remains rigid.
The present invention adopts a rather different approach. It also employs a sealing material based on a SnO--ZnO--P.sub.2 O.sub.5 sealing glass frit that is modified by a mill addition. However, it utilizes a mill addition that increases the material set point, rather than exhibiting non-viscoelastic behavior. Thus, it does not impart a relatively constant viscosity-temperature relationship as a seal is reheated. Rather, it increases the effective set point so that a seal remains relatively rigid at a bakeout temperature of 330.degree. C.